1. Field of the Invention
The present invention relates to a processing device, an image reading device, an image forming apparatus, and a processing method.
2. Description of the Related Art
A scanner includes, for example, a photoelectric conversion element that performs a photoelectric conversion on light that is reflected by an original document; an analog processing unit that performs an operation, such as an A/D conversion, on an analog image signal that is output from the photoelectric conversion element; an image correcting unit that performs various types of correction on a digitally converted image data; and an image transferring unit that transfers image data to a subsequent stage. In accordance with recent requirements for power saving, CMOS sensors have drawn attention as photoelectric conversion elements instead of conventional CCDs, and they have been already used in digital still cameras in general ways.
A CMOS process is used for CMOS sensors, and multiple functions, such as a timing generator (TG) or an analog processing unit, can be integrated. As is the case with scanners, cameras that use CMOS sensors are operated by using a battery, or the like; therefore, there is a very high requirement for power saving in order to enable a long period of operation. Thus, there is a technology in which, as it is no problem if the operation of a CMOS sensor is stopped during a non-capturing time, called a standby state, the operating frequency is decreased during that time so that the power consumption is reduced.
Furthermore, Japanese Patent Application Laid-open No. 2000-324403 discloses a solid-state imaging device in which, during a standby operating mode, on the basis of a dividing-ratio control signal that is obtained by decoding serial data from a serial-data generating unit by using a decoder, a frequency divider circuit divides an atomic clock from an oscillation circuit into eight and sets it as a reference clock.
The conventional technologies for reducing the electric power by decreasing the frequency are effective for CMOS sensors that serially output image signals and that are operated at a high speed by increasing the frequency so as to transfer signals during a specified period. However, in the case of CMOS sensors that output image signals in parallel, the frequency of the drive clock for a pixel circuit is originally about several kHz that is about one hundred times as low as that of conventional CMOS sensors, and decreasing the frequency is less effective for a reduction in the electric power. Furthermore, there is a problem in that, although the current of several dozens of pA flows through a single pixel circuit due to the drive clock, the current of several dozens of μA constantly flows through a bias circuit that is provided in each pixel circuit regardless of the frequency, and this power consumption is dominant.
Therefore, it is desirable to provide a processing device, an image reading device, an image forming apparatus, and a processing method that can achieve a reduction in the power consumption during the standby mode and also achieve a speed-up of a return from the standby mode to the operating mode.